1. Field of the Invention
The present invention relates generally to pumps which employ magnetic suspension and rotation means to pump blood, and more particularly to a magnetically suspended and rotated blood pump that has no mechanical bearings or seals and has a pump means which is magnetically supported radially and axially.
2. Description of the Prior Art
The use of rotary pumps (i.e. axial, centrifugal, mixed flow) to pump fluids and in particular blood is well known by those skilled in the art. A rotary pump, in general, consists of an outer housing, with inlet and outlet ports, and an impeller mounted on a shaft (with mechanical bearings and seals) within the outer housing for rotation about an axis. Mechanical bearings are susceptible to wear and premature failure and can generate sufficient heat and mechanical stresses to cause unacceptable blood damage. Shaft seals are also susceptible to wear and heat generation, which can lead to leakage, blood clot formation, bearing seizure, and bacterial growth. Examples of rotary pumps utilizing shaft mounted impellers with bearings and seals are disclosed in Reich et. al. U.S. Pat. No. 4,135,253; Possell U.S. Pat. No. 4,403,911; Moise U.S. Pat. No. 4,704,121; and Dorman U.S. Pat. No. 4,927,407.
Numerous pumps have been designed to circumvent the above problems by employing a lubricant flush of rotary pump mechanical bearings. Examples of such pumps are disclosed in Carriker et al. U. S. Pat. No. 4,944,722 and Wampler et al. U.S. Pat. No. 4,846,152. These types of pumps can have several problems including not having the ability to be fully implantable due to the need for a percutaneous supply line and external reservoir to achieve bearing flushing. Also the potential for infection and leakage exists due to the flushing fluid and percutaneous lines. In addition the mechanical bearings can still require replacement after time because they directly contact other pump structures during operation.
By employing a rotary fluid pump with a magnetically suspended impeller, all of the above mentioned problems can be avoided. Examples of such pumps are disclosed in Bramm et al. U.S. Pat. No. 5,326,344; Olsen et al. U.S. Pat. No. 4,688,998 and Moise U.S. Pat. No. 4,779,614. A problem which can be associated with all of the cited inventions is that a single gap is employed for both the blood flow pathway through the pump and for the magnetic suspension and rotation of the impeller. These two functions have directly opposing requirements on the size of the gap. As a blood flow pathway, the gap should be large to avoid blood damage. As a magnetic suspension and rotation gap, the gap should be small to minimize the size of the magnetic suspension and rotation components and also to allow for efficient use of energy to achieve impeller suspension and rotation. Consequently, for these types of pumps, any gap size selected can result in an undesirable compromise between blood damage, device size, and energy requirements. Another problem which can be associated with all of the cited inventions is that the magnetic fields created by the permanent magnets in the described pumps can change over time, with variations in temperature, and due to the influence of external ferromagnetic objects. Such changes in the magnetic fields can cause the equilibrium point of the pump rotors to change and can possibly cause the rotors to contact the stators. Contact between the pump rotors and stators can result in blood damage, pump damage, and inadequate pumping.
Examples of pumps having separate gaps for primary blood flow and impeller rotation are disclosed in Golding et al. U.S. Pat. No. 5,324,177 and Golding et al. U.S. Pat. No. 5,049,134. However, these pumps also use the rotation gap to implement hydrodynamic suspension bearings for the rotor. Such hydrodynamic bearings can subject the blood to excessive shear stresses which can unacceptably damage the fragile components of the blood. Additionally, the Golding et. al. pumps place the stationary magnetic components inside a center-bore of a rotating assembly. Such configurations generally cause the mass and rotational inertia of the rotating assembly to be larger than those in a system in which the stationary magnetic components are placed around the outer surface of the rotating assembly. Rotating assemblies having large masses and rotational inertias can be undesirable because the axial and radial bearing elements must be made relatively large in order to maintain proper alignment of the rotating assembly during shock, vibration, and acceleration.
The flow rate of blood pumps that are capable of creating negative inlet pressures must be dynamically adjusted to match the blood flow rate into the ventricle of the heart, typically the left ventricle. If too little flow is produced by the blood pump, the tissues and organs of the body may be inadequately perfused, and the blood pressure in the left ventricle will increase--potentially causing excessive pulmonary pressure and congestion. Conversely, if the flow rate of the blood pump is too high, excessive negative pressure may be created in the left ventricle and in the inlet to the pump. Excessive negative blood pressure is undesirable for the following reasons: 1) Unacceptable levels of blood damage may be caused by cavitation; 2) The pump may be damaged by cavitation; 3) The walls of the ventricle may collapse and be damaged; and 4) The walls of the ventricle may collapse and block the blood flow pathway to the pump.
By employing a control system to dynamically control the flow rate of the pump to avoid excessive negative blood pressure the above mentioned problems can be avoided. One example of such a control system is disclosed in Bramm et al., U.S. Pat. No. 5,326,344. Bramm describes a method of dynamically controlling the flow rate of a pump based on a signal derived from a single pressure sensor located within the pump inlet. One problem which can be associated with such a pressure sensing system is the difficulty in achieving long-term stability of such a sensor, particularly in light of the relatively low pressures (0 to 20 mm Hg) that must be resolved and the hostile environment in which the sensor is operated. Another problem which can be associated with such a pressure sensing system is that the effect of changing atmospheric pressure can cause inaccurate sensing of the pressure needed to properly control the pump.
The natural heart normally creates pulsatile blood flow. Artificial blood pumps that do not create pulsatile flow may cause thrombosis and may inadequately perfuse the organs and tissues of the body. The speed of rotary blood pumps can be varied to create pulsatile flow. Examples of pumps that can vary their speed to create pulsatile flow are disclosed in Isaacson et al. U.S. Pat. No. 5,211,546, Findlay U.S. Pat. No. 5,174,726, and Reich et al. U.S. Pat. No. 4,135,253. One problem that can be associated with varying the speed of the pumps as cited in all of the inventions is if the speed variation is excessive, the blood flow in the pump may become too low and/or reverse direction. If the blood flow in the pump becomes too low the blood may be damaged by constant shearing of the blood pool in the pump. If the blood flow in the pump reverses direction the ventricle may not be adequately unloaded.
Accordingly, there is a need for a blood pump which overcomes the aforementioned problems that can be associated with conventional blood pumps and also a system of dynamically controlling such a blood pump to avoid the previously described problems that can occur with control systems using pressure sensors.